Think of our genes as a code that translates into a finished human being, much like a coded manuscript would translate into a readable text. Now imagine what that text might look like if you went in and covered up various words and phrases so they couldn't be translated. The finished text might be better because of this editing, but it could also be worse or even unreadable. It all depends on what words were kept out of the final copy.

­This is where epigenetics comes into play. The word literally means "above the genome" and ­relates to the changes that occur between the genome and the phenotype. Epigenetic changes don't alter the genes, but they do affect the way they're expressed.

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There are several different kinds of epigenetic changes, but the one we understand the best is methylation. This process involves carbon and hydrogen bundles (CH3) called methyl groups, which bind to the DNA and essentially cover up genes so they can't activate, much like the covered-up phrases in our coded manuscript. Some of those inactive genes could cause disease. In fact, an estimated 50 percent of the reasons for a given disease can be attributed to genetic factors [source: Bhattacharya]. Others parts of the genome, such as tumor-suppressing genes, help to prevent cancer. Epigenetic changes can alter the balance, though. These changes can occur due to several different environmental causes, from the contents of our diet to how stressful our childhood was. To learn more about these changes, read How Epigenetics Works­.

So thanks to the Human Genome Project, we know where all these genes are, but we don't know which genes are expressed in different tissues and what chemical changes switch them on and off. This is where HGP's successor comes in. In 2003, scientists from the United Kingdom'sWellcome Trust Sanger Institute in Cambridge and the biotechnology company Epigenomics formed the Human Epigenome Project (HEP) with the intent of mapping the way methyl groups affect DNA in the human genome. If successful, HEP could enable doctors to better diagnose diseases and advance the field of pharmacogenetics by allowing researchers to develop drugs capable of directly changing the way genes are expressed.

The group set out to map methylation patterns in the human genome, using 200 samples from major human tissues. They were committed to defining methylation variable positions (MVPs) on the X chromosome, Y chromosome and chromosomes 1 through 22. So far, they've completed chromosomes 6, 20 and 22 and plan to continue to map the chromosomes in batches and release them to the public 120 days after each batch has been completed. In recent findings, HEP scientists have observed that DNA methylation remains more stable over the course of an individual's life than previously thought.

Researchers at HEP still have a long way to go toward achieving their goal of mapping the human epigenome, but they hope to gain additional funding and attract involvement from even more researchers. In 2008, the United States government threw its hat into the epigenetic ring, allocating $190 million for the National Institutes of Health's (NIH)'s Roadmap Epigenomics Program. The NIH also awarded grants of up to $12 million to U.S. epigenome mapping centers, epigenomics data analysis and coordination projects, technology development in epigenetics and the discovery of important epigenetic marks in mammalian cells [source: NIH]